Looking up a little history on the Lotus 25&49 I am getting a blonde moment and need a idiots guide plus in laymans terms regarding these two cars.
Basically Wikapaedia says the 25 was a revolutionary design with the first fully stressed monocoque.Whereas the 49 was built around the Cosworth DFV and was the first to feature the engine as a stress member.
Please explain anybody.
Thanks!

Easy to explain if I understand it. I'm going from memory. The Lotus 25 was the first monocoque chassis. The motor bolted into the rear of chassis, and the suspension bolted to the chassis. In other words the chassis extended to the rear, and everything bolted to it.

The Lotus 49 was different. It didn't have a rear part of the chassis. The motor was used in place of the rear part of the chassis. The motor bolted to the bulkhead, and through brackets the suspension bolted to the motor.

Everybody until the Lotus 25 came along used a tubular spaceframe chassis. The Lotus 25 was a big 'U' shaped piece of hollow aluminium. They filled it with fuel, bolted the engine on at the back, added wheels etc. and ran it. It was much stiffer than normal tubular spaceframe chassis, but as Ron Tauranac from Brabham proved, a well designed spaceframe chassis, with all the tubes connecting at the right places, could still be as stiff and light as a monocoque chassis like the Lotus 25.

The Lotus 49 added extra stiffness to the monocoque chassis by using the engine block itself (the Cosworth DFV) to bolt on the suspention pickup points, rather than a tubular spaceframe 'cradle' in which the engine was mounted. The added stiffness aided handling, but as Tauranac proved again, wasn't essential for a light, good handling car if your spaceframe was well designed. The 1969 Brabham BT26a used a Cosworth DFV but in the spaceframe cradle originally intended for the Repco V8, and it handled better than Lotus' own 49b. So, in my opinion, using the engine as a stressed member wasn't such a gamechanger as everyone is now led to believe. Of course, it did make creating a competitive chassis easier, as a lot of the 1970's garagisti proved.

“They filled it with fuel, bolted the engine on at the back”
I think that if you were to write “They filled it with fuel, bolted the engine in at the back” it describes it rather better”, otherwise, fine .
“

I thought the Lotus 49 was based on the Lotus 43 which used the BRM H-16 engine as a stressed member with the suspension bolted to it, I was looking at Jim's '66 Watkins Glen GP winning car at Goodwood earlier this year and it looked remarkably similar to the 49 parked next to it, apart from the enormous engine bolted on the back

And according to Wikipedia:

"The 43 chassis was an excellent design let down by a poor powerplant, and design elements of it were used in the design of its 1967 successor, the far more successful Lotus 49, including the use of the engine as a stressed structural member which bore weight and to which the rear suspension was attached."

I'll dig out my camera and upload a couple of pics shortly... edit here we go:

Yes, the Lotus 43 was sort of based on the Lotus 38 Indy car. It wasn't as bad as most guys think. It was certainly heavier, and not as reliable as the 49, but it wasn't slow. It only raced 4 times. It won one of the races, and qualified on the front row in 3 of the races. Of course this was due to the talent of Jimmy. I'm sure he would have won most of the races in 66 if he had the 49.